The prior art digital hybrid is designed to be coupled between a receive path and a transmit path of a transmitter/receiver equipment and includes a digital filter producing a replica signal of an echo signal and a subtracter circuit to subtract said replica signal from said echo signal which appears on said transmit path in response to an input signal applied to said receive path. The input, echo and replica signals are sampled digital signals and each replica signal sample is the sum of individual terms each constituted by the product of a digital filter coefficient and of a factor which is function of said input signal.
The above-cited article gives no information about how the coefficients of the filter included in the echo canceller are determined. If the echo canceller is adaptive, the filter coefficients may be calculated as described in commonly assigned Belgian Pat. No. 896,089 (B. Aschrafi 1-20 corresponding to laid open German patent application DE No. 32 08 214). However, such calculations necessitate the use of additional relative complex circuits coupled to the transmitter/receiver equipment. In some cases, the additional complexity and expense is not considered justified. An alternative possible solution is to calculate a fixed set of filter coefficients in such a way that the digital filter wherein they are used provides a replica signal which satisfactorily cancels the echo signal under certain predetermined conditions, e.g., when forming part of telecommunication system, the line used in the transmitter/receiver equipment has a length within a predetermined range of line lengths. Obviously, that alternative solution is not satisfactory when the line length is not within the predetermined range, which can be the case in practice.